Microstructural and chemical processes interacted in the transformation of a hornblende-biotite tonalite to a mylonite in an approximately 28-m wide shear zone on the margins of the Cerro de Costilla complex, Baja California, México. Deformation in the tonalite at the edge of the shear zone was initiated by cleavage slip in biotite, leading to wispy, discontinuous foliae. Fracturing of the load-supporting plagioclase framework occurred at the edges and tips of biotite grains. With increasing strain, these foliae became linked via fine-grained aggregates involving fragmental and recrystallized biotite, recrystallized quartz, fragmental plagioclase, minor recrystallized myrmekite, and minor fragmental hornblende, titanite and epidote. The amount of strain accumulation increases markedly across a sharp reaction front approximately 1 m from wall-rock orthogneisses. The front is characterized by metasomatic alteration, which resulted in the disappearance of hornblende in favor of biotite and quartz. Stronger deformation resulted in biotite-quartz-plagioclase 'beards' behind resistant plagioclase clasts. In the most highly strained tonalite, overall grainsize reduction was accompanied by a steady decrease in modal plagioclase, and increases in modal biotite and quartz. Weakening and strain localization were accommodated by: (1) breakdown of the load-bearing plagioclase framework; (2) development of continuous biotite-rich foliae; (3) flow of quartz into lenticular foliae; (4) elimination of strong hornblende, accompanied by modal increase of weaker biotite; (5) dissolution, transport and re-deposition of biotite and quartz components in lower shear-strain sites; and (6) minor contributions from fine-grained aggregates of other minerals. These processes are probably common to many polymineralic shear zones, and are important for initiating and maintaining localized deformation in Earth's crust.